In many industrial uses, a plurality of threaded connecting elements consisting, for example, of pins are employed for fastening a component removably to an appliance.
This is particularly true of the pressurized vessels used in industry, especially the nuclear industry, for example the vessels of nuclear reactors, or the chemical or petrochemical industry, where a cover is fastened removably to a vessel so that it is possible to perform periodic maintenance in the vessel.
For this purpose, the cover is fastened to the vessel by means of pins which are screwed into tapped holes provided in the upper part of the said vessel which penetrate into open-ended bores provided in the flange of the cover and corresponding to the tapped holes.
These pins are usually in somewhat large numbers and relatively close to one another, and therefore the removal and refitting of the cover require the positioning, unscrewing and screwing of all these pins in addition to their tensioning.
In order to place all the pins simultaneously in the open-ended bores of the cover and in the tapped holes of the vessel, the pins are arranged on a supporting member which, as regards the vessel of a nuclear reactor, consists, for example, of a rigid ring, on which the pins are uniformly distributed in polar symmetry.
Until now, the pins were centered in relation to the open-ended bores of the cover and to the tapped holes of the vessel has been obtained by vertically moving the supporting member which is positioned axially in relation to the axis of the cover, and which is guided by a set of rails arranged externally on the upper part of the cover, itself being mounted on the vessel.
It is known that the production of the cover and the vessel allows for production tolerances and that assembling these elements together requires the presence of functional plays.
Moreover, the tapped holes located on the vessel have a position which is diametral in relation to the principal axis of the vessel.
Consequently, because there is no single reference, the cover occupies a particular position independent in relation to the vessel.
Thus, ideal positioning, i.e. perfect superposition of the axis of a pin with the axis of the corresponding tapped hole occurs only by coincidence. This is unsatisfactory and unacceptable for repetitive operations carried out, for example, in a confined environment.